Pool Filters
How do swimming pools stay blue? Most people will correctly assume that it's from the chlorine, or other sanitizer, in the water. But how do they stay clear? To those who have never maintained a swimming pool, the answer isn't so easy to assume. It may be surprising to find out that there is a whole filtration system behind the scenes. Here we will show the many different types of filters used to keep swimming pools sparkling.
NOTICE: Pool Filters can run under very high pressure. They are very dangerous and can cause severe injury or even death. Use caution when maintaining, operating, or repairing. Never have your head or any other body parts above the filter while it is operating.
Sand
The most common filters used by commercial pools are Sand Filters. This is because they're easy to maintain, they last long, and the media that filters all of the particles from the water is Sand, which is very inexpensive.
Sand Filters are simple in design. They are large, round vessels made of fiberglass and filled with pool grade sand. They are air-tight and water-tight with a water inlet, a water outlet and a pressure relief valve with a pressure gauge.
Water is pumped into the inlet which is sprayed over the top of the sand. As the water falls through the sand, the sand grains catch most of the particles in the water. When the water reaches the bottom of the vessel the pressure from the pump pushes it through special finger-like grates called Laterals. These laterals are designed to only allow water through them and not sand. The water flows through the laterals, out through the filter outlet, then eventually back into the pool, clear and particle free.
Over time, the particles caught from the water by the sand will build up and will increase the pressure inside of the filter. This decreases the effectiveness of the filter media. The filter media must be regularly flushed out by running the filter in "Backwash" mode, which runs the water in the opposite direction through the filter, to flush out all of the dirt, debris, and other particles. Sand filters will have special valves attached at the inlet and outlet which gives you the option to run the filter in Backwash mode.
Your pool professional should backwash the pool often (at least twice per week) to keep the sand loose, clean, and effective.
Sand is a popular option for filters because it lasts a very long time, but nothing lasts forever. After a few years of regular use, sand can actually be less effective than it was when it was new. This is due to the sand grains changing shape by erosion, also by material such as cotton from clothing. This causes the sand to become "clumpy" and "clay-like." Good filter sand should be loose and the grains should fall apart from each other relatively easily.
With bad, clumpy filter sand, water will not flow through the loose grains. Instead it will dig channels straight through the sand all the way down to the laterals. This means that instead of the water flowing uniformly through the sand grains, having all of the dirt, particles etc. caught by the grains, it flows down through a channel without any filtration whatsoever. This water will then return to the pool, just as dirty as when it entered the filter in the first place.
A way that you can "rejuvenate" the sand, and get rid of any channels dug into it, is by simply stirring the sand by hand to loosen it.
If even after stirring and loosening the filter sand, the pool does not seem to stay clear, it may be time to replace the sand. The average life of pool grade sand is 5 years. After 5 years it's generally a good idea to have the sand replaced with new sand. Have a pool professional remove and discard the old sand and replace it with new sand. This should make the filter run just like new and you should see the pool clear up rapidly.
Diatomaceous Earth (D.E.)
D.E. Filters are a popular option for many pool owners for being extremely effective at removing particles from pool water using a very fine filter media known as Diatomaceous Earth.
DIatomaceous Earth is a chemically-inert powder made from the fossils of organisms called "diatomes." These fossils are microscopic and porous, like very small sponges. They catch particles and microscopic debris from water. This powder is very fine and does not react chemically to pool water, making it a great option for use as a filter media.
D.E. Filters are cylindrical vessels made of fiberglass or stainless steel. Inside of the vessel, arranged in a spiral pattern, are a series of plastic rib-like structures, covered in a special type of cloth. These are called the filter grids. They're designed to catch all of the D.E. powder on the outside of the cloth, allowing only water to pass through.
When D.E. filters are first installed, water, with D.E. powder added to it, is pumped into the filter inlet. The powder-rich water runs evenly over the outer surfaces of the filter grids. The cloth over the grids catches all of the D.E. powder and holds it in place. After the D.E. powder is added, any water that runs through the powder covered grids will be very effectively filtered and will return back to the water clear and particle free.
While being great at filtering most debris etc from the water, they will eventually get full and need to be backwashed. After backwashing, new D.E. powder will need to be added to the system again for them to work as intended. D.E. powder is most easily added to the system via a skimmer suction inlet.
A very common occurrence with D.E. filters is when the D.E. powder does not get caught on the filter grids and instead sprays into the pool from the jets. This happens because the cloth on one or more of the grids has torn and the water, instead of being filtered through the media, flows through the holes and goes back into the pool. Any D.E. powder you attempt to add to the filter will go straight through these holes and will create a mess in the pool. The filter will no longer be effective and the pool will not stay clear. The grids must all be replaced when this occurs. Individual grids can be replaced one by one but the old grids, despite being undamaged, can become torn very soon after reassembling the filter, which results in constantly disassembling the filter to replace newly damaged grids. For this reason it's a much better idea to replace the whole set at one time. It will save a lot of time and headaches later on.
Cartridge Filters
Cartridge Filters use special cartridges to filter pool water. These cartridges require no additional media as the material of the cartridge itself filters the particles from the water. These cartridges were not designed to be "backwashed" which means they must be manually removed from the filter vessel and rinsed from the outside to remove built up particles, debris, etc.
The inability to remove particles and debris build up from the filter in a practical manner (E.G. "backwashing" for a sand filter) makes cartridge filters a less than ideal option for commercial pools. They are more commonly used for water features such as decorative fountains.
Any system, whether for a pool or fountain, that uses cartridge filters, should be checked often for flow rate. If the water flow starts to slow down, and the pressure inside the filter starts to go up, it's time to have the cartridges cleaned.
The cartridges are cleaned simply by removing them from the filter vessel and spraying them with a water hose. If the cartridges are very heavily coated with dirt, you can use a solution of muriatic acid and water to dissolve heavy dirt buildup. The cartridges should return to almost white in color. They should then be replaced just as they were found inside of the filter vessel, with any and all brackets in place.
While the cartridges are being cleaned, they should be thoroughly checked for holes and any other damage. Damaged cartridges must be replaced.